Highly polarized electrons from GaAs–GaAsP and InGaAs–AlGaAs strained-layer superlattice photocathodes

Abstract

GaAs-GaAsP and InGaAs-AlGaAs strained-layer superlattice photocathodes are presented as emission sources for highly polarized electron beams. The GaAs-GaAsP cathode achieved a maximum polarization of 92(±6)% with a quantum efficiency of 0.5%, while the InGaAs-AlGaAs cathode provides a higher quantum efficiency (0.7%) but a lower polarization (77(±5)%). Criteria for achieving high polarization using superlattice (SL) photocathodes are discussed based on experimental spin-resolved quantum efficiency spectra.

Elimination of local thickness modulation in GaAs/GaAsP strained superlattices for high spin-polarization photocathodes

“…In the GaAsP layer on the GaAs substrate, some cracks along [110] and [1][2][3][4][5][6][7][8][9][10] directions were observed as shown in Figure 3 (a). Except the cracks areas, the surface of GaAsP layer shows relatively flat.…”

“…So the highly spin-polarized electrons are selectively excited by circularly polarized photons only from the heavy hole mini-band to the conduction mini-band, and are then emitted from a negative electron affinity surface. Photocathodes based on a GaAs/GaAsP strained superlattice [4,5] and an AlInGaAs/AlGaAs strained superlattice [6] provide high spin-polarizations over 90%. In our previous study [5,7] we have newly developed a super-high-brightness transmission-type photocathode based on the GaAs/GaAsP strained superlattice.…”

Improvement of Quantum Efficiency in Transmission-Type Spin-Polarized Photocathode

“…In order to avoid absorption in the inter-layer, Al x Ga 1-x As (x=0.4) was used instead of GaAs that was necessary to induce tensile strain in the GaAsP buffer layer [11]. Al 0.4 Ga 0.6 As (E g : 1.9 eV) is transparent to the pump laser light (hν: 1.59 eV) and the lattice constant of AlGaAs is almost the same as that of GaAs.…”

“…In the strained SL structure, the heavy-hole and light-hole mini-bands are split due to both the elastic strain effect and the quantum confinement effect. The reflection-type photocathodes based on a GaAs/GaAsP strained SL [11] and an AlInGaAs/AlGaAs strained SL [12] produced a high spin polarization (SP) of approximately 90%.…”

Highly Effective Polarized Electron Sources Based on Strained Semiconductor Superlattice with Distributed Bragg Reflector

“…The enlarged valence band splitting results in a high initial electron polarization in the conduction band under excitation by circularly polarized light. The progress in design and fabrication of strained SL in the past years results in the development of electronic sources with maximal electronic polarization higher 90% [2][3][4]. However even in the best photocathode samples, quantum efficiency (QE) does not exceed 0.8%.…”